Plastic bottles are formed with concave bottoms in order to form an outer bottom rim by which the bottles can stand upright on a shelf. The plastic material in the bottles is made as thin as possible in order to minimize the quantity of plastic material so that the bottles can be produced at a cost which is competitive with other commercially produced bottles. Flat bottoms made of such thin plastic materials would bulge from the pressure of carbonated beverages and render bottles with such bottoms unstable when placed upright on a shelf causing the bottles to rock or fall over. Bottoms with concave center portions offer an improvement in shelf stability of the bottles, but the competition to reduce cost by using less material in the bottle has lead to many different types of reinforcements for concave bottoms to avoid bulging of the thinner concave bottoms.
One example of a prior art reinforced concave bottom is illustrated in U.S. Pat. No. 4,502,607 which shows a depending solid rib formed along the mold parting line at the pinched off end of a preform. This rib offers substantial support for the concave portion of the bottom to prevent bulging of the concave portion. However, bottles formed with such depending solid ribs are subject to standing instability due to bulging from internal bottle pressures in bottom rim portions which are disposed on opposite sides of the solid ribs.
Another example of prior art bottom reinforcement is shown in U.S. Pat. No. 4,108,324 which has eight radially spaced hollow convex ribs extending from the concave portion to the rim portion of the bottom. While these hollow convex ribs result in increased resistance to bulging compared to a non-reinforced concave bottom, they do not produce the degree of bulge resistance that a solid rib formed at the pinch off seam can produce.